BIOSYNTHESIS OF METHIONINE FROM HOMOCYSTEINE AND s-METHYLMETHIONINE IN BACTERIA

Abstract

A combination of homocysteine and S-methylmethionine replaces methionine for growth in some bacteria. Cell-free extracts were obtained from methionine auxotrophs of Aerobacter aerogenes, as well as from wild-type A. aerogenes and Escherichia coli strain K 12, which produce methionine from homocysteine with S-methylmethionine as the methyl donor. The amount of methionine obtained is as high as 90% of the theoretical yield. Incorporation into methionine of S35 and C14H3 from S-methylmethionine in cell-free extracts is consistent with the proposed over-all reaction for methylation of homocysteine: Homocysteine + S-methylmethionine[forward arrow] 2 methionine. S-adenosylmethionine can replace S-methylmethionine as a methyl donor only to a very limited extent under these conditions. None of the other known methyl donors show any activity with growing cells or cell-free extracts. Cystathionine can replace homocysteine partially in growing cells and cell-free extracts, presumably because it is converted to homocysteine.